Coproxy is a project mainly written in PHP, it's free.
php proxy based on http://code.google.com/p/php5rp
.htaccess
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f [NC]
RewriteCond %{REQUEST_FILENAME} !-d [NC]
RewriteCond %{REQUEST_URI} !^/index.php
RewriteRule ^(.+)$ index.php/$1 [QSA]
index.php
require('./coProxy.php');
$proxy = new coProxy('public/url','private/url');
if(!$proxy->valid('proxy.lock'))
die(file_get_contents('./na.html'));
$proxy->run();